The propagation of the eukaryotic genome depends upon the initiation of DNA replication from numerous origins. Origin activity requires the binding and activation of a pre-Replicative Complex (pre-RC). In multi-cellular eukaryotes, however, it is not known how certain regions on chromosomes are selected for pre-RC binding and activation;a DNA consensus has yet to emerge. Moreover, the genomic sites that act as origins, and the time that they initiate during S phase, can change during development, but it is not known what determines this developmental specificity. We have employed a model system based on developmental amplification of Drosophila eggshell (chorion) genes to investigate origin identity and the regulation of genome duplication. Our previous results indicated that nucleosome acetylation regulates the developmental specificity of chorion and other origins in the ovary. We propose to extend this work by investigating the mechanism by which nucleosome modifications influence the activity of origins using genetic and molecular methods. An important aspect of this proposal is that it takes advantage of methods in Drosophila to address the role of chromatin in developmental specificity of origin activity. Our long term goals are to describe the full repertoire of chromatin modifications at origins, to reveal the mechanism by which chromatin alters origin activity, and to discover how these modifications are targeted to specific origins in different cell types in development. This investigation should lead to important new insights into the epigenetic control of genome duplication during development. Lay description: A complete and accurate copy of DNA must be made each cell division. To accomplish this large task, the cell starts copying from numerous "origins". How certain sites in the genome are chosen to be origins is not understood. This proposal investigates how the structure of chromosomes regulates where DNA replication starts in different cells. The results will provide a deeper understanding for how defects in this process contribute to human cancers, leading to better diagnosis and therapies.